1. Field of the Invention
The present invention relates to a laminated structure, in which opposed electrodes are provided, to be used in piezoelectric pumps, piezoelectric actuators, ultrasonic transducers and so on, and further relates to a method of manufacturing the laminated structure and the piezoelectric actuator.
2. Description of a Related Art
Laminated structures in each of which dielectric layers of piezoelectric materials or the like and electrode layers are alternately stacked, are utilized, in various uses such as capacitors, piezoelectric pumps, piezoelectric actuators and ultrasonic transducers. As such laminated structure, not only a single-layer structure in which electrodes are formed on both ends of a dielectric layer, but also a multi-layer structure in which plural dielectric layers and electrode layers are alternately formed is used. By connecting the stacked plural layers in parallel, the capacitance between electrodes of the entire laminated structure can be enlarged even having a small size, and thereby, rise in electric impedance can be suppressed. In recent years, with the developments of MEMS (micro electromechanical systems) related devices and equipment, elements each having such a laminated structure have been microfabricated still further and packaged more densely.
By the way, in order to efficiently apply a voltage to the dielectric layer, it is important that the dielectric layer and the electrode layer are in close contact. In the case where an element employing a piezoelectric material as a dielectric is fabricated, generally, platinum (Pt) having low reactivity to a piezoelectric material is used as an electrode material. However, platinum provides low adhesion to a piezoelectric material, a substrate, or the like. Accordingly, it has been practiced to further provide a titanium (Ti) layer to make an electrode to have a double structure in order to bring platinum into close contact with an under layer material.
However, in the manufacturing process of an element, generally, firing of a piezoelectric material is performed, and, at that time, the adhesive layer of titanium or the like is oxidized by oxygen transmitted through the platinum. As a result, bonding force between the piezoelectric material and the adhesive layer becomes weak and the electrode peels off the piezoelectric material. Further, sometimes the electrode peels off the piezoelectric material due to difference between coefficients of thermal expansion of the piezoelectric material and the electrode material.
In order to prevent the separation at an interface between a piezoelectric film and an electrode, Japanese Patent Application Publication JP-P2001-156351A discloses a laminated structure electrode including a piezoelectric film formed on an Si substrate, on which an electrode film is disposed, according to a gas deposition method, characterized in that the electrode film is made of one or more kind of material(s) and has a thickness of more than 0.15 μm. In JP-P2001-156351A (page 1, FIG. 1), an electrode in which three kinds of metals of platinum, iridium (Ir) and tantalum (Ta) are stacked is used as a lower electrode of the piezoelectric film. That is, by inserting an iridium layer, through which oxygen is difficult to be transmitted, between a platinum layer and a tantalum layer as an adhesive layer to a substrate underneath, the oxygen transmitted through the platinum layer is prevented from penetrating the tantalum layer.
The electrode having such structure is effective when used as a lower electrode of a structure having a single-layered piezoelectric material layer. However, in the case where the electrode having such structure is used as an internal electrode of the laminated structure in which plural piezoelectric material layers are stacked, oxygen penetrates not only from the upper piezoelectric material layer but also from the lower piezoelectric material layer, and thereby, the electrode similarly peels off.